Rats fed on a tryptophan-deficient diet develop zonular and nuclear lens opacities. From an analysis of tissue sections it appears that the zonular opacity corresponds to a region in the lens cortex of swollen fiber cells with pycnotic nuclei. These observations suggest that in some way the deficiency of tryptophan leads to the abnormal maturation of fiber cells. Preliminary work using immunological techniques has enabled us to localize the major groups of proteins in the growing rat lens. Whether there is a similar distribution of these proteins in the abnormal lenses of tryptophan deficient rats, particularly in the swollen cortical fiber, is presently under investigation.